323 research outputs found
Energy input and HI spin temperatures in low pressure regions
Two recent (unpublished) HI emission/absorption studies carried out with good sensitivity using the Arecibo 21 cm beam are discussed. One study (Colgan, Salpeter and Terzian) looked for high velocity clouds of our own Galaxy in absorption in the directions of 63 of the brightest continuum sources reachable with the Arecibo telescope. HI emission mapping in the neighborhood of these directions was also carried out. The other study (Corbelli and Schneider) looked for absorption along lines of sight to about 50 weaker sources which pass within a few diameters of nearby disk galaxies. Neither study detected any absorption
The Onset of the Cold HI Phase in Disks of Protogalaxies
We discuss a possible delay experienced by protogalaxies with low column
density of gas in forming stars over large scales. After the hydrogen has
recombined, as the external ionizing UV flux decreases and the metal abundance
increases, the HI, initially in the warm phase (T\simgt 5000 K), makes a
transition to the cool phase (T\simlt 100 K). The minimum abundance
for which this phase transition takes place in a small fraction of the Hubble
time decreases rapidly with increasing gas column density. Therefore in the
``anemic'' disk galaxies, where is up to ten times smaller than for
normal large spirals, the onset of the cool HI phase is delayed. The onset of
gravitational instability is also delayed, since these objects are more likely
to be gravitationally stable in the warm phase than progenitors of today's
large spiral galaxies. The first substantial burst of star formation may occur
only as late as at redshifts and give a temporary high peak
luminosity, which may be related to the ``faint blue objects". Galaxy disks of
lower column density tend to have lower escape velocities and a
starburst/galactic fountain instability which decreases the gas content of the
inner disk drastically.Comment: TeX file, 24 pages, 4 figures available upon request from
[email protected], to appear in The Astrophysical J. (Sept. 1
Hi Spin Temperatures and Heating Requirements in Outer Regions of Disk Galaxies
ABSRACT:We show how to use 21-cm emission and absorption studies to estimate
the heat inputs to the neutral gas in low pressure environments, such as in
outer disks of spiral galaxies, in galactic halos or in intergalactic space.
For a range of model parameters we calculate the gas kinetic and spin
temperatures ( and ) and the relation between and the heat
input to the gas. We outline the conditions for a ``two phase medium'' to
exist. We find that although can be much smaller than , is
always K for column densities greater that
cm. This excludes the possibility that relevant HI masses at the
periphery of galaxies are invisible at 21-cm in emission. The outermost
interstellar gas in a disk galaxy is more directly affected by external
processes and in this paper we estimate the intensity of the extragalactic
background at energies close to 0.1 keV by comparing our theoretical results
with HI emission/absorption studies. We take into account the possibility that
some energy produced in the inner regions affects the energy balance in outer
regions. We find that in the absence of any other local heat source QSO
dominated background models are still compatible with the spin temperature
limits derived for the two best documented HI emission/absorption studies in
outer regions.Comment: 24 pages, 8 figures ARCETRI-PR-93-2
The Molecular Gas Distribution and Schmidt Law in M33
The relationship between the star formation rate and surface density of
neutral gas within the disk of M33 is examined with new imaging observations of
CO J=1-0 emission gathered with the FCRAO 14m telescope and IRAS HiRes images
of the 60 micron and 100 micron emission. The Schmidt law, Sigma_SFR ~
Sigma_gas^n, is constructed using radial profiles of the HI 21cm, CO, and far
infrared emission. A strong correlation is identified between the star
formation rate and molecular gas surface density. This suggests that the
condensation of giant molecular clouds is the limiting step to star formation
within the M33 disk. The corresponding molecular Schmidt index, n_{mol}, is
1.36 +/- 0.08. The star formation rate has a steep dependence on total mass gas
surface density, (Sigma_{HI}+Sigma_{H_2}), owing to the shallow radial profile
of the atomic gas which dominates the total gas surface density for most radii.
The disk pressure of the gas is shown to play a prominent role in regulating
the molecular gas fraction in M33.Comment: 19 pages + 5 figures. Accepted for publication in Ap
Stellar structures in the outer regions of M33
We present Subaru/Suprime-Cam deep V and I imaging of seven fields in the
outer regions of M33. Our aim is to search for stellar structures corresponding
to extended HI clouds found in a recent 21-cm survey of the galaxy. Three
fields probe a large HI complex to the southeastern (SE) side of the galaxy. An
additional three fields cover the northwestern (NW) side of the galaxy along
the HI warp. A final target field was chosen further north, at a projected
distance of approximately 25 kpc, to study part of the large stellar plume
recently discovered around M33. We analyse the stellar population at R > 10 kpc
by means of V, I colour magnitude diagrams reaching the red clump. Evolved
stellar populations are found in all fields out to 120' (~ 30 kpc), while a
diffuse population of young stars (~ 200 Myr) is detected out to a
galactocentric radius of 15 kpc. The mean metallicity in the southern fields
remains approximately constant at [M/H] = -0.7 beyond the edge of the optical
disc, from 40' out to 80'. Along the northern fields probing the outer \hi
disc, we also find a metallicity of [M/H] = -0.7 between 35' and 70' from the
centre, which decreases to [M/H] = -1.0 at larger angular radii out to 120'. In
the northernmost field, outside the disc extent, the stellar population of the
large stellar feature possibly related to a M33-M31 interaction is on average
more metal-poor ([M/H] = -1.3) and older (> 6 Gyr). An exponential disc with a
large scale-length (~ 7 kpc) fits well the average distribution of stars
detected in both the SE and NW regions from a galactocentric distance of 11 kpc
out to 30 kpc. The stellar distribution at large radii is disturbed and,
although there is no clear correlation between the stellar substructures and
the location of the HI clouds, this gives evidence for tidal interaction or
accretion events.Comment: 13 pages, 13 figures. Accepted for publications in Astronomy and
Astrophysics; minor revisions of the tex
HI clouds in the proximity of M33
Neutral hydrogen clouds are found in the Milky Way and Andromeda halo both as
large complexes and smaller isolated clouds. Here we present a search for Hi
clouds in the halo of M33, the third spiral galaxy of the Local Group. We have
used two complementary data sets: a 3^o x 3^o map of the area provided by the
Arecibo Legacy Fast ALFA (ALFALFA) survey and deeper pointed observations
carried out with the Arecibo telescope in two fields that permit sampling of
the north eastern and south-western edges of the HI disc. The total amount of
Hi around M33 detected by our survey is M. At least 50%
of this mass is made of HI clouds that are related both in space and velocity
to the galaxy. We discuss several scenarios for the origin of these clouds
focusing on the two most interesting ones: dark-matter dominated gaseous
satellites, debris from filaments flowing into M33 from the intergalactic
medium or generated by a previous interaction with M31. Both scenarios seem to
fit with the observed cloud properties. Some structures are found at anomalous
velocities, particularly an extended HI complex previously detected by Thilker
et al. (2002). Even though the ALFALFA observations seem to indicate that this
cloud is possibly connected to M33 by a faint gas bridge, we cannot firmly
establish its extragalactic nature or its relation to M33. Taking into account
that the clouds associated with M33 are likely to be highly ionised by the
extragalactic UV radiation, we predict that the total gas mass associated with
them is > 5 x 10^7 M. If the gas is steadily falling towards the M33
disc it can provide the fuel needed to sustain a current star formation rate of
0.5 M yr.Comment: 16 pages, 19 figures. Accepted for publication in A&
Radial dependence of the dark matter distribution in M33
The stellar and gaseous mass distributions, as well as the extended rotation curve, in the nearby galaxy M33 are used to derive the radial distribution of dark matter density in the halo and to test cosmological models of galaxy formation and evolution. Two methods are examined to constrain the darkmass density profiles. The first method deals directly with fitting the rotation curve data in the range of galactocentric distances 0.24 64 r 64 22.72 kpc. Using the results of collisionless \u3b4 cold dark matter numerical simulations, we confirm that the Navarro-Frenkel-White (NFW) dark matter profile provides a better fit to the rotation curve data than the cored Burkert profile (BRK) profile. The second method relies on the local equation of centrifugal equilibrium and on the rotation curve slope. In the aforementioned range of distances, we fit the observed velocity profile, using a function that has a rational dependence on the radius, and we derive the slope of the rotation curve. Then, we infer the effective matter densities. In the radial range 9.53 64 r 64 22.72 kpc, the uncertainties induced by the luminous matter (stars and gas) become negligible, because the dark matter density dominates, and we can determine locally the radial distribution of dark matter. With this second method, we tested the NFW and BRK dark matter profiles and we can confirm that both profiles are compatible with the data, even though in this case the cored BRK density profile provides a more reasonable value for the baryonic-to-dark matter ratio
The Stellar IMF in Very Metal-Deficient Gas
In the context of the star formation through the fragmentation of an
extremely metal-deficient protogalactic cloud, the gravitational collapse of
filamentary gas clouds is explored with H and HD chemistry. It is found by
1D hydrodynamical simulations that the cloud evolution is prescribed mainly by
the initial density () and H abundance (). In
particular, it turns out that the evolution of low-density filaments ( cm) bifurcates at a critical H abundance of , beyond which HD cooling overwhelms H
cooling. The numerical results indicate that the stellar IMF is likely to be
double-peaked and deficient in sub-solar mass stars, where the high mass peak
of the IMF is around or , dependently on the initial
density and H abundance. If the gas in protogalactic clouds is photoionized
by UV radiation or shock-heated, the H abundance could exceed
by H reactions. Then, the high
mass peak would be .Comment: 4 pages, 1 figure, proceedings of New Quests in Stellar Astrophysics:
The link between Stars and Cosmology (eds. M. Chavez, A. Bressan, A. Buzzoni
& D. Mayya, to be published by the Kluwer Academic Publishers
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